Liquid ink printers of the type frequently referred to as continuous stream or as drop-on-demand, such as piezoelectric, acoustic, phase change wax-based, or thermal, have at least one printhead from which droplets of liquid ink are directed towards a recording medium. Within the printhead, the ink is contained in a plurality of ink conduits or channels. Power pulses cause the droplets of ink to be expelled as required from orifices or nozzles at the ends of the channels.
In a thermal ink-jet printer, the power pulse is usually produced by a heater transducer or a resistor, typically associated with one of the channels. Each resistor is individually addressable to heat and vaporize ink in the channels. As voltage is applied across a selected resistor, a vapor bubble grows in the associated channel and initially bulges toward the channel orifice followed by collapse of the bubble. The ink within the channel then retracts and separates from the bulging ink thereby forming a droplet moving in a direction away from the channel orifice and towards the recording medium whereupon hitting the recording medium a dot or spot of ink is deposited. The channel is then refilled by capillary action, which, in turn, draws ink from a supply container of liquid ink.
The ink jet printhead may be incorporated into either a carriage type printer, a partial width array type printer, or a page-width type printer. The carriage type printer typically has a relatively small printhead containing the ink channels and nozzles. The printhead can be sealingly attached to a disposable ink supply cartridge and the combined printhead and cartridge assembly is attached to a carriage which is reciprocated, at a constant speed, to print one swath of information (equal to the length of a column of nozzles), at a time, on a stationary recording medium, such as paper, fabric, or a transparency. After the swath is printed, the paper is stepped a distance equal to the height of the printed swath or a portion thereof, so that the next printed swath is contiguous or overlapping therewith. This procedure is repeated until the entire page is printed. In contrast, the page width printer includes a stationary printhead having a length sufficient to print across the width or length of the recording medium at a time. The recording medium is continually moved past the page width printhead in a direction substantially normal to the printhead length and at a constant or varying speed during the printing process. A page width ink-jet printer is described, for instance, in U.S. Pat. No. 5,192,959, herein incorporated by reference.
Printers typically print information received from an image output device such as a personal computer. Typically, this received information is in the form of a raster scan image such as a full page bitmap or in the form of an image written in a page description language or a combination thereof. The raster scan image includes a series of scan lines consisting of bits representing pixel information in which each scan line contains information sufficient to print a single line of information across a page in a linear fashion. Printers can print bitmap information as received or can print an image written in the page description language once converted to a bitmap consisting of pixel information.
Information printed by a printer can be printed having the grayscale of the received information. The printer can also modify the received bitmap and print the information at a grayscale different than the one received. In either event, it is typical that grayscale printing is performed either by controlling the number of drops per pixel, by controlling the size of the white space between pixels, or both. One known method of improving image quality through grayscale control is to print pixels on a higher resolution grid in both the scan direction and the paper advance direction. Such methods require multiple passes of the printhead so that more drops are placed within a given region of the print medium. These methods also require small and very accurate paper advance steps for accurate placement of the drops on a high resolution grid. Because such methods require multiple passes of the printhead, print speed may be undesirably reduced. In addition, accurate paper advance steps may increase the cost of the printing system as such systems typically require advanced electronic controllers and expensive encoders which can be cost prohibitive.
Various methods and apparatus for printing images with scanning carriage type liquid ink printers have been developed. The following references describe these and other methods and apparatus for liquid ink printing.
U.S. Pat. No. 4,965,593 to Hickman describes a dot printer wherein the spacing of ink jet nozzles of a print head are spaced by an amount greater than the pixel spacing of the printing medium such that adjacent pixels are not printed until the deposited colorant has time to dry.
European Patent Application Publication No.623 473 to Holstun et al, describes increased print resolution in the carriage scan axis of an ink-jet printer. The increased print resolution is achieved by moving the carriage of an ink-jet cartridge in the carriage scan direction to provide a first resolution in that direction which is twice the second resolution in a print media advance direction. Two smaller drops of ink are fired onto each square pixel in a single pass of the cartridge so as to provide, for example, a 600 dpi resolution in the carriage scan axis with a 300 dpi resolution in the media advance direction.